Today's wireless devices often support multiple wireless standards that operate over multiple frequency bands. For example, a cellular phone or handset may be configured to communicate with both second and third generation wireless communications systems using multiple standards (e.g., EDGE/GSM/WCDMA) and frequency bands (e.g., the 900, 1800, and 2100 MHz bands). A wireless local area network device is another common wireless device that may be configured to support multiple standards (e.g., IEEE's 802.11a/b/g standards) and frequency bands.
These multi-band wireless devices include at least one radio frequency (RF) front-end to upconvert a signal for transmission over a wireless link and to down convert a signal received over a wireless link. Each wireless standard typically imposes strict performance requirements that often prevent the use of a single, self-contained RF front-end for all frequency bands supported by a wireless device. Consequently, a multi-band wireless device normally reproduces and customizes several circuit blocks of an RF front-end for each supported band.
The RF front-end typically includes a VCO to generate a local oscillator (LO) signal for frequency upconversion and downconversion. In wireless devices, the VCO is typically implemented using an inductor-capacitor (LC) tank circuit that includes an inductor and one or more capacitors. The frequency of the LO signal generated by the VCO can be controlled by varying the capacitance of the LC tank. A VCO can generally achieve good performance over a small tuning range (e.g., when the frequency bands of interest are within 10 to 20% of each other). However, for a large tuning range the resistive impedance of the LC tank circuit can vary considerably, which results in a relatively large and undesirable variation in the VCO phase noise over the different, supported frequency bands.
To combat phase noise, a wireless device that supports multiple bands that are spaced far apart can employ multiple VCOs. Each VCO can then be designed to achieve good performance for a specific frequency band. However, the use of multiple VCOs for multiple frequency bands increases cost and area in an integrated circuit (IC) implementation.
Therefore, what is needed is a multi-band VCO that has reduced phase noise over a large tuning range to support today's multi-band wireless devices.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.